1. Field of the Invention
The present invention relates to characterization of a conductive plane and more specifically to measuring the resistance of the conductive plane using a multipoint measurement probe.
2. Description of the Related Art
Generally, the resistance of a conductive plane is determined based on single point measurement. However, in some cases, single point measurements may provide an incorrect resistance value for the conductive plane. In particular, single point measurements may not be representative of current in cases where spreading is significant.
FIGS. 1A and 1B illustrate two conductive planes (one generally narrower and one generally broader for a given length. FIG. 1A illustrates an example of single point resistance measurement of generally narrower conductive plane 110. The resistance of conductive plane 110 is measured by measuring the voltage across point A and B. A known current I1 flows through conductive plane 110 and current I1 creates a current field 120 in conductive plane 110. The resistance of conductive plane 110 is measured using the voltage across points A and B, known current I1 and the distance between points A and B. The measurement between points A and B do not take into account the spread of known current I1 across conductive plane 110.
FIG. 1B illustrates another example of single point resistance measurement of the generally wider conductive plane 130. The resistance of conductive plane 130 is determined by measuring the voltage across points C and D. A known current I2 flows through conductive plane 130 and creates a current field 140. The resistance of conductive plane 130 is measured using the voltage across points C and D, known current I2 and the distance between points C and D. The measurement between points C and D do not take into account the spread of known current I2 across conductive plane 130. This measurement gives a lower measured value than FIG. 1A measurements even if sheet resistance is the same, due to spreading out of current flow.
Because conventional single point resistance measurement techniques do not take into account the spread of current across the width of conductive planes, measured resistance of a conductive plane (particularly a wide conductive plane) can be inaccurate. Accordingly, conventional measurement techniques do not measure the resistance of conductive planes per square unit for a uniform current. The accurate resistance measurement of conductive plane is important especially, for semiconductor packaging. A semiconductor package for an integrated circuit can include multiple conductive planes providing peripheral contacts for the integrated circuit. Often resistance measurements based on a single pair of points, whatever the source, do not accurately reflect the current spread on the semiconductor package from the integrated circuit die that is placed at the center of the package to the package capacitors on the periphery. Furthermore, such resistance measurements can be particularly vulnerable to variations in localized electrical characteristics of conductive planes as actually fabricated. In general, variations may trace to geometric or process factors. An incorrect resistance measurement of the package can lead to erroneous design of power distribution for the integrated circuit which can severely affect the performance of the integrated circuit.
Techniques are needed to accurately measure the resistance of a conductive plane.